Detection of Incipient Thermal Damage Of Carbon Fiber/Epoxy Composites Using Fluorescent Thermal Damage Probes

Thesis (Ph.D.)--University of Washington, 2013

Incipient thermal damage (ITD) of carbon fiber/epoxy composites occurs due to short term exposures to temperatures known to cause matrix degradation. ITD is a concern for composite parts because it can significantly reduce mechanical strength of the part, but it cannot be detected by common inspection techniques such as ultrasound and thermography. This paper focuses on using molecular probes whose fluorescence emission changes irreversibly after thermal exposure as a means to quickly and efficiently detect areas of the composite that have been thermally exposed. These probes are referred to as thermal damage probes. In this research, two fluorescent thermal damage probes were designed and synthesized. These probes were called AJNDE16 and AJNDE35. The fluorescence emission behavior of these probes in response to thermal exposure was characterized in several epoxy systems with cure temperatures ranging from room temperature to 177 °C. It was found that the fluorescence of both probes changed with thermal exposure as expected indicating there viability of the probes as sensors for thermal exposure. Several potential issues with utilizing the probes were identified including oxidation of the matrix, temporal stability, the presence of carbon fibers, and the ability to detect localized thermal damage were investigated. Oxidation was found to quench the fluorescence signal, but the signal could easily be restored by removing the oxidation with light sanding. The temporal stability at ambient conditions was found to be excellent with no significant decrease in the signal observed over 18 months. The presence of carbon fibers was found to cause more scattering of the excitation light source, but otherwise did not have much of an effect on the measurements. The probes also exhibited excellent ability to detect a localized thermal exposure. The activation kinetics of both probes was also measured and the probes were found to exhibit a first order reaction rate. Using the kinetic measurements, models were generated for both probes. It was found that using only one probe could indicate that a significant thermal exposure had occurred, but it was not capable of quantifying the amount of thermal exposure. To solve this issue, probes AJNDE16 and AJNDE35 were combined to form a multiplexed probe system. It was found that the fluorescence response of the multiplexed system to thermal exposure could be modeled as the superposition of the fluorescence emission peaks of the individual probes using the kinetic models. Utilizing ratiometric fluorescence measurements of the peak intensities at 540 and 560 nm, the multiplexed system was found to be capable of acting as a time-temperature indicator (TTI) with a response on the temperature and time scale of ITD in CFRP. Using the multiplexed system model response curves for temperatures in the range known to cause ITD were generated. Due to difficulty determining the complete thermal history (time and temperature) of part, the responsive curves were utilized by setting a time or temperature of interest and estimating an effective value of the other parameter. To test the accuracy of the multiplexed system as a TTI, it was applied as a coating to a composite panel and exposed to a localized thermal event. Using the response curves it was found that the estimated parameter (time or temperature) matched very well with thermocouple data from the exposure.